Such a distance controller is described in German Patent Application No. DE 10 2005 009 702 A1, and is part of a driver assistance system, also called an ACC (Adaptive Cruise Control) system, and is used to automatically control the distance from a vehicle traveling ahead to a specified target distance, generally as a function of speed. For this purpose, as a remote region sensor a long-range radar (LRR) sensor is generally provided with which the distances and relative speeds, and also the azimuth angles, of vehicles traveling ahead are measured. On the basis of the azimuth angle, the vehicle traveling immediately ahead in the home lane is then identified, and this vehicle is chosen as the target object for the distance controlling. If no target object is present, controlling takes place to a desired speed selected by the driver.
In the conventional ACC systems, the distance controlling function is generally available only at speeds greater than a certain minimum speed, for example 30 km per hour, because these systems are generally provided for trips on highways or well-finished rural roads, and do not react to standing objects. ACC systems are also under development that have an automatic stop and/or start function, called a stop and go function, making it possible to brake the vehicle automatically to a standstill if, for example when approaching the end of a traffic jam, the radar sensor determines that the target object is stopped.
In some cases, an automatic restart function is also provided, so that the vehicle automatically moves back into motion if the radar sensor determines that the target object has started to move. Under some conditions, for example after the elapsing of a determined standstill time or on the basis of a configuration correspondingly selected by the driver, the system may operate in such a way that the start process is initiated only if the driver has previously received a start indication and has confirmed it by an input operation.
For the stop and go function, in general a more complex sensor system and evaluation algorithm are required, because the system now also has to react to stationary objects, and also because uninterrupted monitoring of the close range is now required, because, for example in congested traffic, persons or other obstacles may be situated in the close range immediately in front of the vehicle. A single LRR is not sufficient for a complete monitoring of the close range area, because due to the divergent radar lobe, dead angles result on both sides of the radar lobe.
Therefore, in some cases the LRR is supplemented with two short-range radar (SRR) sensors that monitor the near field.
In the German patent application described above, in contrast, it is proposed for the monitoring of the near field to use ultrasound sensors that are generally already present anyway as a component of an electronic parking assistance system in the vehicle. The ultrasound sensors also make it possible to locate objects and to measure their distance from the vehicle. However, for distance controlling during travel the range of the ultrasound sensors is not sufficient. According to the proposal in the German application, however, the ultrasound sensors can be used to check, before a start process, whether obstacles are situated in the near field in front of the home vehicle. Moreover, because the location areas of the LRR and of the ultrasound sensor system (USS) overlap, in situations in which a target object is situated in the location area of both systems a mutual functional test of the systems is possible. If the ultrasound sensors are to be used in the decision concerning an automatic starting process, this is particularly essential, because deposits of dirt or snow on the ultrasound sensors can easily result in a decrease in sensitivity or a complete occlusion.
In the conventional system, the signal of the ultrasound sensors is evaluated only for the decision as to whether a risk-free starting of the vehicle is possible or not. The actual longitudinal guiding of the vehicle, i.e., the controlling of the acceleration and slowing processes by which the distance to the target object is regulated and stop processes are controlled in such a way that the vehicle comes to a stop at an appropriate distance behind the target object, takes place solely on the basis of the distance signals supplied by the remote region sensor.
However, in the case of very small distances the distance measurements carried out with the aid of the LRR are too imprecise. Therefore, in conventional stop and go systems the stop distance at which the home vehicle is brought to a stop behind the target object is on the order of magnitude of approximately 5 meters, and is thus within a distance range in which an evaluable LRR signal is still present. The restarting takes place only with a relatively low dynamic. In particular, the starting process can be initiated only if the LRR has reliably determined that the vehicle ahead has again started to move. In traffic jam situations on highways or on rural roads, this system behavior is completely acceptable. However, in standing traffic, in view of the limited space and the traffic capacity, limited by the length of the green phases of traffic lights, it would be desirable for the stop distances to be shortened, and for it to be possible to carry out the automatic starting processes more dynamically.